Reengineering plants to be more drought and salt tolerant

John Cushman is a foundation professor in the Department of Biochemistry and Molecular Biology, whose research proposal to the U.S. Department of Energy's Joint Genome Institute has been picked for a partnership under its Plant Flagship Genomes program.

"The goal of the plant flagship program is basically to sequence genomes, or sequence transcriptomes of a series of target crops that are of importance to the mission of the Department of Energy, mainly as bioenergy feedstocks," Cushman said. "And these can also include model species that grow rapidly and are easy to study to better understanding gene function."

Cushman's project proposal, a research effort that started in 2012, focuses on the common or crystalline ice plant and its demonstrated tolerance to stressors such as salinity and drought.

The JGI holds a yearly competition for its Community Science Program for researchers who are exploring solutions to energy and environmental challenges, while also giving them access to high-quality resources to continue their area of research.

Cushman's proposal was one of the 37 selected out of 123 original letters of intent submitted, and 98 full proposals received.

Cushman's lab is looking at the functional genomics of crassulacean acid metabolism or CAM. CAM is a water-conserving photosynthetic pathway that helps plants survive in seasonally arid climates or those with intermittent water supply.

"Our project, and the project of our collaborators at Oak Ridge National Laboratory and the Universities of Liverpool and Newcastle in the United Kingdom, on another model CAM species called Kalanchoe, is to understand CAM," Cushman said. "CAM is present in more than six percent of all vascular plant species across 36 different plant families, so it is a fairly widespread ecological adaptation."

The importance of the ice plant, which originated in the Namibian desert in Africa, is that it is the first reported plant species that could be induced to switch from C3 photosynthesis to CAM following salinity stress or water-deficit treatment.

Most plants use what is known as the C3 pathway to photosynthetically fix atmospheric carbon during the day. However, plants that rely on the water-conserving CAM pathway take up and fix carbon during the night, thereby avoiding water losses that normally occur due to a process called evapotranspiration, which helps keep plants cool during the day. However, daytime transpiration results in water loss through small pores in the leaf surfaces called stomata.

"They open their stomata so carbon dioxide can enter the leaf, and then it gets fixed into sugars and all other compounds that support most of life on Earth," Cushman said. "But because plants transpire to cool themselves, they lose enormous amounts of water."

However, CAM plants limit this water loss by keeping their stomata closed during all or most of the day, and only opening them at night when evapotranspiration is low because it is cooler and the sun is not shining. Thus, CAM plants are five-to-six times more water-use efficient than C3 photosynthesis plants.

The research objectives for Cushman's lab are to understand how the expression of CAM is controlled by environmental stress and the circadian clock. The lab is conducting integrated transcriptome, proteome and metabolome analyses using the ice plant, which is capable of surviving under extremely harsh environmental conditions.

"We have studied the process of gene expression, and so we know exactly which genetics parts are important for doing CAM, and that's why the ice plant is such an important model, and that's why the DOE is interested in it," Cushman said. "So now, we can take those genes and reengineer them back to a C3 photosynthesis plant like wheat or rice, or a woody bioenergy feedstock like poplar, and we hope to make those more water-use efficient."

During the past 30 years, Earth's temperature has begun to rise, and the resulting heat and drought effects are beginning to slow the rate of increasing crop productivity.

"It's simple really, we release carbon dioxide and other greenhouse gases into the atmosphere and the Earth gets hotter," Cushman said. "More heat leads to greater soil drying and more transpirational water loss, both of which in turn lead to greater possibility of drought stress. So, one of the predictions of global warming is that with all of this heating, we are going to need to make more drought-tolerant plants in the very near future."

A microbiologist by training, Cushman received his bachelor's degree from Ursinus College, in Collegeville, Pennsylvania, and his master's and doctorate from Rutgers University - New Brunswick, New Jersey. He has been a part of the College of Agriculture, Biotechnology and Natural Resources at the University of Nevada, Reno since 2000, and also serves as the Director of the Graduate Program in Biochemistry and Molecular Biology.

"I like microbiology but I thought that plants would have more relevance," Cushman said. "And I like stress, so I got into the area of plant stress as I was doing my post-doctoral research. I became interested in inducible systems, so the ice plant was a beautiful system, and that's how this first began."

Cushman's project is ongoing, and he said that it could take several more years to truly learn about the plant and what the research could do. But, with the project's partnership with the Joint Genome Institute, his lab will have access to state-of-the-art resources and facilities to continue this research.